Method and apparatus for reducing coupling between signals in a measurement system

ABSTRACT

A method and an apparatus for separating a composite signal into a plurality of signals is described. A signal processor receives a composite signal and separates a composite signal in to separate output signals. Pre-demodulation signal values are used to adjust the demodulation scheme.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/216,583, filed Mar. 17, 2014, titled “Method and Apparatus forReducing Coupling Between Signals in a Measurement System,” which is acontinuation of U.S. application Ser. No. 12/983,823, filed Jan. 3,2011, now U.S. Pat. No. 8,676,286, titled “Method and Apparatus forReducing Coupling Between Signals in a Measurement System,” which is acontinuation of U.S. application Ser. No. 11/371,242, filed Jan. 23,2006, now U.S. Pat. No. 7,865,222, titled “Method and Apparatus forReducing Coupling Between Signals in a Measurement System,” which is acontinuation of U.S. application Ser. No. 10/615,333, filed Jul. 8,2003, now U.S. Pat. No. 7,003,338, titled “Method and Apparatus forReducing Coupling Between Signals,” the entire contents of which ishereby incorporated by reference.

BACKGROUND

1. Field

The present disclosure relates to the field of signal processing, and,more particularly, relates to multi-channel demodulators fordemodulating mixed signals, such as, for example, signals generated in apulse oximetry system.

2. Description of the Related Art

In many multi-channel measurement and communication systems, crosstalkbetween channels and corruption of data within the channels aresignificant problems. Such problems can arise from variations inmanufacturing tolerances, movement, propagation delays, phase shifts,temperature effects, degradation of components due to age or otherfactors, noise, etc.

A pulse oximetry system is one example of a system where theabove-referenced problems are found. In a pulse oximetry system, bloodoxygen saturation is determined by transmitting pulses ofelectromagnetic energy through a portion of a subject having bloodflowing therein (e.g., through a finger, through an ear lobe, or otherportion of the body where blood flows close to the skin). The pulses ofelectromagnetic energy comprise periodic pulses of red light havingwavelengths of approximately 660 nanometers, for example, and periodicpulses of infrared light having wavelengths of approximately 905nanometers.

After propagating through the portion of the subject, the red pulses andthe infrared pulses are detected by a detector which is responsive tolight at both wavelengths and which generates an electrical signal thathas a relationship to the intensity of the electromagnetic energyincident on the detector. The detector output is a two-channel signalhaving a first signal component corresponding to the detected red pulsesand a second signal component corresponding to the detected infraredpulses.

The two-channel signal is demodulated to recover separate signalscorresponding to the first signal component and the second signalcomponent. However, prior art demodulators are not sufficiently accurateenough to completely separate the two signal components in all cases.Thus, it is not uncommon for the first demodulator output signal(corresponding to the first signal component) to contain residualcomponents of the second signal and vice versa. This crosstalk betweenthe first and second signal components reduces the accuracy of therecovered first and second signals. In multi-channel systems with morethan two channels, crosstalk can occur between all of the channels,again reducing accuracy.

SUMMARY

The present disclosure solves these and other problems by separating acombined multi-channel signal into a plurality of output signals in amanner that reduces crosstalk and other contamination in the pluralityof output signals. In one embodiment, the separator includes amulti-channel demodulator that is first configured using nominal valuesfor the various components in the signal path. In one embodiment, themulti-channel demodulator is further configured using data obtained fromcalibration measurements. In one embodiment, the calibrationmeasurements are made during an initialization period. In oneembodiment, the calibration measurements are made frequently,continuously, or at selected intervals. In one embodiment, calibrationsare performed on at least one of, initialization, on command, onattachment of a new sensor, continuously, and/or interspersed withmeasurements. In one embodiment of a system for measuring one or moreblood constituents, the calibration measurements are made when thesystem detects that a patient has been connected to the system. In oneembodiment, the multi-channel demodulator is further configured atregular intervals by re-running the calibration measurements. In oneembodiment, the multi-channel demodulator comprises an optimizingdemodulator. In one embodiment, crosstalk in the multi-channeldemodulator is reduced by computing an amplitude and/or phase adjustmentof one or more demodulation signals that are provided respectively toone or more mixers.

In one embodiment, an apparatus for measuring blood oxygenation in asubject includes a first signal source which applies a first inputsignal during a first time interval. A second signal source applies asecond input signal during a second time interval. A detector detects afirst parametric signal responsive to the first input signal passingthrough a portion of the subject having blood therein and detects asecond parametric signal responsive to the second input signal passingthrough the portion of the subject. The detector generates a detectoroutput signal responsive to the first and second parametric signals. Asignal processor receives the detector output signal and demodulates thedetector output signal by applying a first demodulation signal to asignal responsive to the detector output signal to generate a firstdemodulator output signal and applying a second demodulation signal tothe signal responsive to the detector output signal to generate a seconddemodulator output signal. In one embodiment, the first demodulationsignal has at least one component comprising a first frequency, a firstphase, and a first amplitude; and the second demodulation signal has atleast one component comprising a second frequency, a second phase, and asecond amplitude. In one embodiment, the first phase and the secondphase are chosen to reduce crosstalk from the first parametric signal tothe second demodulator output signal and to reduce crosstalk from thesecond parametric signal to the first demodulator output signal. In oneembodiment, the first amplitude and the second amplitude are chosen toreduce crosstalk from the first parametric signal to the seconddemodulator output signal and to reduce crosstalk from the secondparametric signal to the first demodulator output signal. In oneembodiment, at least one of the first amplitude, the first phase, thesecond amplitude, and the second phase are chosen to reduce crosstalkfrom the first parametric signal to the second demodulator output signaland to reduce crosstalk from the second parametric signal to the firstdemodulator output signal.

In one embodiment, at least one of the first amplitude, the first phase,the second amplitude, and the second phase is determined by turning offone of the first and second signal sources and measuring the crosstalkbetween one of the parametric signals and the non-corresponding outputsignal.

One embodiment includes a method of reducing crosstalk between twosignals generated by applying a first pulse and a second pulse tomeasure a parameter. The first pulse and the second pulse are appliedperiodically at a repetition rate defining a period. The first pulse isgenerated during a first interval in each period and the second pulse isgenerated during a second interval in each period. In one embodiment,the second interval is spaced apart from the first interval. In oneembodiment, the second interval overlaps at least a portion of the firstinterval. The first and second pulses produce first and secondparametric signals responsive to the parameter. The first and secondparametric signals are received by a detector which outputs a compositesignal responsive to the first and second parametric signals. The methodincludes applying a first demodulation signal to the composite signal togenerate a first demodulated output signal. The first demodulationsignal includes at least one component having at least a first amplitudeand a first phase. The method further includes applying a seconddemodulation signal to the composite signal to generate a seconddemodulated output signal. The second demodulation signal includes atleast one component having at least a second amplitude and a secondphase. The method further includes lowpass filtering the firstdemodulated output signal to generate a first recovered output signalresponsive to the first parametric signal, and lowpass filtering thesecond demodulated output signal to generate a second recovered outputsignal responsive to the second parametric signal. The method alsoincludes choosing at least one of the first phase, the first amplitude,the second phase, and the second amplitude to reduce crosstalkcomponents in the first recovered output signal and the second recoveredoutput signal. In one embodiment, the method also includes choosing thefirst phase and/or the second phase to reduce crosstalk components inthe first recovered output signal and the second recovered outputsignal.

In one embodiment, the first phase and the second phase are chosen byapplying a first light pulse pattern during a first time period andmeasuring the first recovered output during the first time period as afirst calibration output, and measuring the second recovered outputduring the first time period as a second calibration output. The methodincludes applying a second light pulse pattern during a second timeperiod and measuring the first recovered output during the first timeperiod as a third calibration output and measuring the second recoveredoutput during the second time period as a fourth calibration output. Themethod further includes computing the first phase and the second phasefrom at least the first calibration output, the second calibrationoutput, the third calibration output, and the fourth calibration output.

In one embodiment the first phase is computed from a ratio of the firstcalibration output and the second calibration output.

In one embodiment, the first demodulation signal includes a sum of afirst demodulation component having a first amplitude and a seconddemodulation component having a second amplitude. The seconddemodulation component is in quadrature with the first demodulationcomponent and the act of choosing the first phase involves choosing thefirst amplitude and the second amplitude. In one embodiment thequadrature components are sinusoidal and cosinusoidal.

In one embodiment, the first demodulation signal includes a sum of asinusoidal component having a first amplitude and a cosinusoidalcomponent having a second amplitude. The first amplitude and the secondamplitude are chosen by a least squares minimization of an errorcorresponding to the crosstalk. In one embodiment, the error isintegrated over a time period corresponding to an integer number ofcycles of the sinusoidal component.

In one embodiment, a first demodulation signal is applied to a compositesignal having first and second coefficients to generate a firstdemodulated signal. The first demodulation signal includes a firstcomponent having a first amplitude and a second component having asecond amplitude. The first and second components being in quadrature.The second amplitude has a predetermined relationship to the firstamplitude. The predetermined relationship is selected to cause the firstdemodulated signal to have lower frequency components that include aprimary component corresponding primarily to the first desired componentand a residual component corresponding to the second component. Thefirst demodulated signal is lowpass filtered to generate a first outputsignal. At least one of the first amplitude and the second amplitude areadjusted to reduce the residual component with respect to the primarycomponent.

In one embodiment, a pulse oximetry system includes a modulation signalgenerator. The modulation signal generator generates a first modulationsignal including a first pulse at a repetition frequency having a firstduty cycle. The modulation signal generator generates a secondmodulation signal including a second pulse which also repeats at therepetition frequency and having a second duty cycle. The second pulsecan be non-overlapping with respect to the first pulse, or the secondpulse can partially or completely overlap the first pulse. The first andsecond pulses include a plurality of components wherein a firstcomponent has a frequency corresponding to the repetition frequency anda second component has a second frequency corresponding to twice thefirst frequency. A first transmitter emits electromagnetic energy at afirst wavelength in response to the first pulse. A second transmitteremits electromagnetic energy at a second wavelength in response to thesecond pulse. A detector receives electromagnetic energy at the firstand second wavelengths after passing through a portion of a subject. Thedetector generates a detector output signal responsive to the receivedelectromagnetic energy. The detector output signal includes a signalcomponent responsive to attenuation of the electromagnetic energy at thefirst wavelength and a signal component responsive to attenuation of theelectromagnetic energy at the second wavelength. A first demodulatormultiplies the detector signal by a first demodulation signal andgenerates a first demodulated output signal. A second demodulatormultiplies the detector signal by a second demodulation signal andgenerates a second demodulated output signal. A configuration moduleconfigures the first demodulation signal and the second demodulationsignal to substantially separate the first demodulator output and thesecond demodulator output.

In one embodiment, the configuration module selects a phase relationshipbetween the first demodulation signal and the second demodulationsignal.

In one embodiment, the configuration module configures the firstdemodulation signal and the second demodulation signal using, at leastin part, data obtained during a calibration period. In one embodiment,the calibration data includes first and second calibration datacorresponding to the first and second demodulated output signals duringa first time period, and third and fourth calibration data correspondingto the first and second demodulated output signals during a second timeperiod. In one embodiment, the second transmitter is turned off duringthe first time period, and the first transmitter is turned off duringthe second time period.

In one embodiment, the configuration module configures the firstdemodulation signal and the second demodulation signal by adjustinginitial parameters that define the first demodulation signal and thesecond demodulation signal. The configuration module adjusts the initialparameters using, at least in part, the calibration data obtained duringa calibration period.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be described below in connection with theaccompanying figures.

FIG. 1 is a block diagram of a multi-channel processing system that usesfeedback from one or more outputs to configure the operation of a signalseparator that separates a composite signal into a plurality of outputsignals.

FIG. 2 is a block diagram of a two-channel signal processing system todetermine blood oxygen saturation in a subject, wherein illumination isprovided by back-to-back Light-Emitting Diodes (LEDs).

FIG. 3 is a block diagram of a multi-channel signal processing system todetermine blood constituents (e.g., oxygen saturation) in a subject,wherein illumination is provided by N diodes or illumination sources.

FIG. 4 is a block diagram of a specific embodiment of the multi-channelprocessing system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a topology of a multi-channel measurement or communicationsystem 100. The system 100 has a signal combiner 103 for combining oneor more input signals S₁ . . . S_(N) into a composite signal and asignal separator 104 for separating the composite signal into one ormore output signals Ŝ₁ . . . Ŝ_(M) The output signals Ŝ₁ . . . Ŝ_(M) caninclude estimates of the input signals S₁ . . . S_(N). The input signalsS₁ . . . S_(N) are corrupted by pre-combination distortion 101-102respectively, and, optionally, by combination distortion in the signalcombiner 103. The combiner 103 combines the N input signals into acomposite signal (or composite signals). The combiner 401 can combinesignals by addition, subtraction, multiplication, division, modulation,non-linear processes, linear processes, estimation, combinationsthereof, etc. The composite signal is provided through a communicationchannel to the separator 104. The composite signal is distorted bycommunication channel distortion 110. The separator 104 separates thecomposite signal into M output signals, where M can be less than N,greater than N, or equal to N. In one embodiment, the separator alsoprovides one or more additional output signals corresponding toestimates of other signals, such as, for example, noise signals, errorsignals, etc.

Due to errors in the system 100, the output signals Ŝ₁ . . . Ŝ_(M) aretypically not exact copies of the input signals, but rather areestimates of the input signals. The accuracy of these estimates is ameasure of system performance. The pre-combination distortion 101-102,the combiner distortion, and/or the channel distortion 110 tend tointroduce crosstalk between the channels and thereby corrupt the outputsignals. The pre-combination distortion 101-102, combiner distortion,and the channel distortion 110 can be caused by variations inmanufacturing tolerances, delay, movement, temperature effects,degradation of components due to age or other factors, noise, etc.

A module 105 is provided to configure the separator 104 to improve thequality of the separation function and thereby improve the quality ofthe output signals. One or more of the output signals from the separatorare provided to the module 105 to provide feedback regarding the qualityof the output signals and/or feedback regarding the operation of theseparator 104. The module 105 uses feedback from one or more of theoutput signals Ŝ₁ . . . Ŝ_(M) (and, optionally, the output signals{circumflex over (n)}₀ . . . {circumflex over (n)}_(K)) to monitor thequality of the separation function and to provide control information tocontrol the operation of the separator. In one embodiment, the module105 is configured by using configuration data obtained from the combiner103. Such configuration data can be obtained by calibration proceduresthat test the operation of the combiner 103 before or during system use.

In one embodiment, the module 105 configures demodulators in the signalseparator 104 using, at least in part, calibration data obtained duringa calibration period. For example, in one embodiment involving a twochannel system, the calibration data includes first and secondcalibration data corresponding to the first and second output signalsduring a first time period, and third and fourth calibration datacorresponding to the first and second demodulated output signals duringa second time period. In one embodiment, the second transmitter isturned off during the first time period, and the first transmitter isturned off during the second time period. In one embodiment, the module105 configures the first demodulation signal and the second demodulationsignal by adjusting initial parameters that define the firstdemodulation signal and the second demodulation signal. Theconfiguration module adjusts the initial parameters using, at least inpart, the calibration data obtained during a calibration period.

FIG. 2 is a block diagram of a two-channel signal processing system 200that fits the general topology shown in FIG. 1. The system 200 isconfigured to determine one or more blood constituents in a subject,such as, for example, a human subject. In the example presented, themeasurements are performed on a portion of the subject, such as a finger202 illustrated in FIG. 2A. An LED modulation circuit 204 drives a pairof back-to-back light emitting diodes (LEDs) 206, 208 by applying aperiodic signal to the two light emitting diodes 206, 208. Light fromthe diodes 206, 208 passes through the finger 202 and is detected by adetector 250. An output from the detector 250 is provided to a signalprocessing block 270. A control output from the signal processing block270 is provided to the LED modulation circuit 204. The signal processingblock 270 also provides outputs Ŝ₁ and Ŝ₂ corresponding to the lightdetected from the diodes 206, 208, and, optionally, output signals{circumflex over (n)}₀ . . . {circumflex over (n)}_(K) corresponding toestimates of noise or other signals.

In one embodiment, the LED 206 is selected to emit electromagneticenergy in the red visible light range, and has a wavelength of, forexample, approximately 660 nanometers. The LED 208 is selected to emitelectromagnetic energy in the infrared range, and has a wavelength of,for example, approximately 905 nanometers. The LED modulation circuit204 supplies current to activate the LEDs 206 and 208. Each LED isactivated for a time period τ which can be different for the differentLEDs. The pulses from the LEDs 206 and 208 repeat with a periodicity T.

FIG. 3 is a block diagram of a multi-channel signal processing system300 that also fits the topology shown in FIG. 1. Like the system 200,the system 300 is configured to determine blood oxygen saturation orother blood constituents in a subject, such as, for example, a humansubject. In FIG. 3, an LED modulation circuit 314 drives N diodes, whereN is two or greater, thus allowing greater flexibility than the system200. In FIG. 3, the diodes 206 and 208 are shown, along with an N'thdiode 309, it being understood that the diode 309 is omitted if N=2. TheLED modulation circuit 314 is configured to allow the diodes 206, 208,and 309 to be driven independently such that the diodes can be driven atseparate times or in overlapping time periods if desired. Light from thediodes 206, 208, 309 passes through the finger 202 and is detected bythe detector 250. The output from the detector 250 is provided to asignal processing block 371. A control output from the signal processingblock 371 is provided to the LED modulation circuit 314. The signalprocessing block 371 also provides outputs S₁ through S_(M), where M isgreater than or equal to one, but need not be equal to N.

FIG. 4 shows one embodiment of an adjustable multi-channelmodulator/demodulator system 400. The system 400 is based on thetopology of the system 100 and can be used in a system for measuringblood constituents (e.g., pulse oximetry, carboxyhemoglobin, etc.) asshown in FIGS. 2 and 3. In the system 400, an input S₁(t) and amodulation input M₁(t) are provided to a first modulator 491. A signalinput S_(N)(t) and a modulation input M_(N)(t) are provided to an N^(th)modulator 402.

The photodetector 250 is modeled as an adder 405. The outputs of themodulators 491 and 402 are added together in the adder 405, in thepresence of noise n(t) to generate a composite signal M(t) where:

S(t)=S ₁(t)M ₁(t)+ . . . +S _(N)(t)M _(N)(t)+n(t)  (1)

The S(t) signal output of the adder 405 (i.e., the output of thedetector 250) is applied to the input of a signal-processing block 410.Within the signal-processing block 410, the signal S(t) is passedthrough an amplifier 497 and through an analog bandpass filter 498. Theanalog bandpass filter 498 provides anti-aliasing and removal of lowfrequency noise and DC. The desired signal components in the signalsS_(i)(t) are frequency shifted by the operation of the modulationsignals M_(i)(t) and are passed by the analog bandpass filter 498.

The output of the analog bandpass filter 498 is sampled by ananalog-to-digital converter 499 and converted therein to digital signalsand provided to an input of an optional decimation block 420.

The filtered (and, optionally, decimated) signal S(t) is sampled toproduce a sampled-data signal S(k) that is provided to the first inputof the first mixer 424, to the first input of the N'th mixer 412, and tothe first input of a noise channel mixer 413. A first demodulatingsignal D₁(k) is provided to a second input of a first mixer 424 from asignal generator 431. The N^(th) demodulating signal D_(N)(k) isprovided to an N^(th) mixer 412 from an output of a signal generator432. The noise demodulating signal D₀(k) is provided to the noisechannel mixer 413 from an output of a signal generator 441. A controlinput to each of the signal generators 431, 432, and 441 is provided bythe output of the adjuster algorithm 450. In yet another embodiment, theadjuster algorithm 450 may also be controlled by other signal processingelements downstream of the signal processor 400.

The outputs of the mixers 413, 424, and 412 are provided as respectiveinputs to decimation blocks 440, 430, and 434 respectively. Each of thedecimation blocks 440, 430, and 434 has a control input provided by theoutput of the adjuster algorithm block 450. The output of the decimationblock 440 is an estimate of the signal n(t) and it is provided to aninput of the adjuster algorithm block 450. In an alternate embodiment,the signal estimates Ŝ_(i)(k) are also provided to the adjusteralgorithm block 450.

An output of the decimator 430 is a signal Ŝ₁(k), which, as discussedabove, is an estimate of the signal S₁(k) (where S₁(k) corresponds to asampled-data representation of S₁(t)). Likewise, the output of thedecimation block 434 is an estimate of the signal S_(N)(t). As shownabove, the selection of the demodulating signals D_(i)(t) for i=0 . . .N in accordance with the present disclosure substantially reduces oreliminates the effects of noise in the output signals Ŝ_(i)(k) and n(k),and also substantially reduces or eliminates crosstalk between thesignals.

When the system 400 is used in connection with a blood constituentmeasurement system as shown in FIGS. 2 and 3 the red LED 206 provides alight intensity represented as I_(RD), and the infrared LED 208 providesa light intensity represented as I_(IR). The effects of turning the LEDs206, 208 on and off on a periodic bases are modeled by the firstmultiplier or modulator 290 which applies a first modulation signalM₁(t) to the red light intensity to generate a modulated red signalI_(RDMOD)(t) and by a second multiplier the modulator 292 which appliesa second modulation signal M₂(t) to the infrared light intensity togenerate a modulated infrared signal I_(IRMOD)(t). The modulated lightred signal and the modulated infrared signal are applied to the finger202, or other body portion, as described above. The blood in the finger202 has a volume and scattering components, which vary throughout eachcardiac cycle. The blood carries oxygen and other materials therein. Theoxygen content is a function of both the blood volume and theconcentration of the oxygen in the blood volume. The concentration ofthe oxygen in the blood volume is generally measured as blood oxygensaturation for reasons which are described in full in U.S. Pat. Nos.5,482,036 and 5,490,505, both of which are hereby incorporated byreference in their entirety. As further described in the two referencedpatents, the blood oxygen saturation is determined by comparing therelative absorption of the red light and the infrared light in thefinger 202. The comparison is complicated by the noise caused bymovement, ambient light, light scattering, and other factors. Thesignals S₁(t) and S₂(t) represent the effect of the time-varying volumeand scattering components of the blood in the finger 202 on the redlight and the infrared light, respectively, passing through the finger202 from the LEDs 206, 208 to the detector 250.

As shown in FIG. 4, a set of N+1 signals S_(i)[k] i=1 . . . N, and n(k)are sampled at a desired sample rate. The signals are combined accordingto the formula:

S(k)=M ₁(k)S ₁(k)+ . . . +M _(N)(k)S _(N)(k)+n(k)  (2)

In one embodiment, each of the decimators 420, 440, 430, and 434includes a digital lowpass filter and a sample rate compressor. In oneembodiment, the characteristics of the digital lowpass filters (e.g.,the number of filter coefficients and values of the filter coefficients)and the sample rate compression factor of each decimator are fixed. Inone embodiment, the characteristics of the digital lowpass filters(e.g., the number of filter coefficients or values of the filtercoefficients) and the sample rate compression factor of each decimatorare provided by the adjustment algorithm 450. The signal generators 431,432 and 441 generate the demodulation sequences for the demodulators424, 412, and 413 respectively. The demodulation sequences produced bythe signal generators 431, 432 and 441 are controlled by the adjusteralgorithm 450.

In one embodiment, the adjuster algorithm 450 adjusts thepre-demodulation decimation rate R₁ (in the demodulator 420), and thepost-demodulation decimation rate R₂ (in the demodulators 430, 434 and440) according to the noise in the noise estimate {circumflex over(n)}(k) and (optionally) according to the signals Ŝ_(i)(k). The productR₁R₂ is the total decimation rate from the signal S(k) at the output ofthe A/D converter 499 to the signals Ŝ_(i)(k) at the output of thesignal processing block 400. The adjuster algorithm may adjust R₁ and R₂such that the product R₁R₂ varies, or the adjuster algorithm may adjustR₁ and R₂ such that the product R₁R₂ is substantially constant.Typically, the adjuster algorithm will keep the R₁R₂ product constant sothat the signal processing blocks downstream of the signal processor 400will operate at a substantially constant sample rate.

In one embodiment, the adjuster algorithm 450 adjusts the demodulationsignals D_(i)(k) to reduce or eliminate crosstalk. In one embodiment,the adjuster algorithm 450 reduces crosstalk by configuring thedemodulators, as discussed in more detail below.

One skilled in the art will recognize that the lowpass filters providedin connection with the decimation blocks can provide other filterfunctions in addition to lowpass filtering. Thus, for example, thelowpass filters 420, 430, 440, and 450, and the decimators 420, 430,434, and 440 can provide other filter functions (in addition to lowpassfiltering) such as, for example, bandpass filtering, bandstop filtering,etc. Moreover, the post-demodulation decimation rate R₂ need not be thesame for each output channel. Thus, for example, in FIG. 4, thedecimator 440 can have a first decimation rate R₂=r₁ while thedecimators 430 and 434 have a second decimation rate R₂=r₂.

The demodulators above are described in terms of digital signalprocessing on sampled data. Thus, the demodulator signals are writtenD_(i)(k). The demodulators and the filtering associated with thedemodulators can be done in using analog processing (using time-domaindemodulator signals D_(i)(t)) or on sampled data signals (usingdigital-domain demodulator signals D_(i)(k)). For convenience, thefollowing development describes the demodulator signals primarily in thetime domain, with the understanding that the modulators can beimplemented using digital signal processing or analog processing.

The characteristics of the demodulation signals D₁(t) and D₂(t) affecthow much crosstalk is seen in the output signals. In an diagonal system,that is, when the demodulator has been diagonalized, there is, ideally,no crosstalk. The first output signal Ŝ₁(t) is an estimate (orapproximation) to the signal S₁(t). Similarly, the second output signalŜ₂ (t) is an estimate (or approximation) to the signal S₂(t). When thecomposite signal S(t) is a linear combination of the signals S_(i)(t),then the relationship between the signals S₁(t) and the signalsŜ_(i)(t). When M₁=cos ωt, M₂=sin ωt, n(t)=0, and there is no distortion(e.g., no pre-combination, combiner, or channel distortion) then:

$\begin{matrix}{{{S(t)} = {{{S_{1}(t)}\cos \; \omega \; t} + {{S_{2}(t)}\sin \; \omega \; t}}}{{Then}\text{:}}} & (3) \\{{{\hat{S}}_{1}(t)} = {{LP}\left\lbrack {{D_{1}(t)}{S(t)}} \right\rbrack}} & (4) \\{{{{\hat{S}}_{2}(t)} = {{LP}\left\lbrack {{D_{2}(t)}{S(t)}} \right\rbrack}}{{If}\text{:}}} & (5) \\{{D_{1}(t)} = {2\; \cos \; \omega \; t}} & (6) \\{{{D_{2}(t)} = {2\; \sin \; \omega \; t}}{then}} & (7) \\\begin{matrix}{{{D_{1}(t)}{S(t)}} = {{2{S_{1}(t)}\cos^{2}\omega \; t} + {2{S_{2}(t)}\sin \; \omega \; t\; \cos \; \omega \; t}}} \\{= {{S_{1}(t)} - {{S_{1}(t)}\cos \; 2\; \omega \; t} + {{S_{2}(t)}\sin \; 2\; \omega \; t}}}\end{matrix} & (8)\end{matrix}$

After lowpass filtering to remove the terms with a frequency of 2ωt andhigher

Ŝ ₁(t)=S ₁(t)  (9)

Similarly for

$\begin{matrix}{{{{\hat{S}}_{2}(t)} = {{LP}\left\lbrack {{D_{2}(t)}{S(t)}} \right\rbrack}}{then}} & (10) \\\begin{matrix}{{{D_{2}(t)}{S(t)}} = {{2\; {S_{1}(t)}\sin \; \omega \; t\; \cos \; \omega \; t} + {2\; {S_{2}(t)}\sin^{2}\omega \; t}}} \\{= {{S_{2}(t)} - {{S_{2}(t)}\cos \; 2\omega \; t} + {{S_{1}(t)}\sin \; 2\omega \; t}}}\end{matrix} & (11)\end{matrix}$

After lowpass filtering to remove the terms with a frequency of 2ωt

Ŝ ₂(t)=S ₂(t)  (12)

In the above analysis, it was assumed that there are no time delays orphase shifts in the signal S(t), and thus, configuration is relativelystraightforward

When an unknown delay (or phase error) is introduced, then the signalsare no longer diagonal. Consider, for example, the situation when adelay Δ is introduced into the composite signal. Then:

S(t)=cos ω(t−Δ)S ₁(t−Δ)+sin ω(t−Δ)S ₂(t−Δ)

It then follows that:

$\begin{matrix}{{{\hat{S}}_{1}(t)} = {{LP}\left\lbrack {2\; \cos \; \omega \; {t\left( {{\cos \; \omega \; \left( {t - \Delta} \right){S_{1}\left( {t - \Delta} \right)}} + {\sin \; {\omega \left( {t - \Delta} \right)}{S_{2}\left( {t - \Delta} \right)}}} \right)}} \right\rbrack}} \\{= {{{LP}\left\lbrack {2\; \cos \; \omega \; {t\left( {{\cos \; \omega \; t\; \cos \; {\omega\Delta}} + {\sin \; \omega \; t\; \sin \; {\omega\Delta}}} \right)}{S_{1}\left( {t - \Delta} \right)}} \right\rbrack} +}} \\{{{LP}\left\lbrack {2\; \cos \; \omega \; {t\left( {{\sin \; \omega \; t\; \cos \; {\omega\Delta}} - {\cos \; \omega \; t\; \sin \; {\omega\Delta}}} \right)}{S_{2}\left( {t - \Delta} \right)}} \right\rbrack}} \\{= {{\cos \; {\omega\Delta}\; {S_{1}\left( {t - \Delta} \right)}} - {\sin \; {\omega\Delta}\; {S_{2}\left( {t - \Delta} \right)}}}}\end{matrix}$

The above equations can be expressed in matrix form as:

$\begin{matrix}{{\begin{bmatrix}{{\hat{S}}_{1}(t)} \\{{\hat{S}}_{2}(t)}\end{bmatrix} = {\begin{bmatrix}{\cos \; {\omega\Delta}} & {{- \sin}\; {\omega\Delta}} \\{\sin \; {\omega\Delta}} & {\cos \; {\omega\Delta}}\end{bmatrix}\begin{bmatrix}{S_{1}\left( {t - \Delta} \right)} \\{S_{2}\left( {t - \Delta} \right)}\end{bmatrix}}}{Then}} & (13) \\{\begin{bmatrix}{S_{1}\left( {t - \Delta} \right)} \\{S_{2}\left( {t - \Delta} \right)}\end{bmatrix} = {\begin{bmatrix}{\cos \; {\omega\Delta}} & {\sin \; {\omega\Delta}} \\{{- \sin}\; {\omega\Delta}} & {\cos \; {\omega\Delta}}\end{bmatrix}\begin{bmatrix}{{\hat{S}}_{1}(t)} \\{{\hat{S}}_{2}(t)}\end{bmatrix}}} & (14)\end{matrix}$

The above equation can be expressed as

$\begin{matrix}{\begin{matrix}{\begin{bmatrix}{S_{1}\left( {t - \Delta} \right)} \\{S_{2}\left( {t - \Delta} \right)}\end{bmatrix} = {\begin{bmatrix}{\cos \; {\omega\Delta}} & {\sin \; {\omega\Delta}} \\{{- \sin}\; {\omega\Delta}} & {\cos \; {\omega\Delta}}\end{bmatrix} \cdot {{LP}\begin{bmatrix}{{D_{1}(t)}{S(t)}} \\{{D_{2}(t)}{S(t)}}\end{bmatrix}}}} \\{= {{LP}\left\lbrack {{\begin{bmatrix}{\cos \; {\omega\Delta}} & {\sin \; {\omega\Delta}} \\{{- \sin}\; {\omega\Delta}} & {\cos \; {\omega\Delta}}\end{bmatrix}\begin{bmatrix}{D_{1}(t)} \\{D_{2}(t)}\end{bmatrix}}{S(t)}} \right\rbrack}} \\{= {{LP}\left\lbrack {\begin{bmatrix}{{\overset{\_}{D}}_{1}(t)} \\{{\overset{\_}{D}}_{2}(t)}\end{bmatrix}{S(t)}} \right\rbrack}}\end{matrix}{where}\begin{matrix}{{{\overset{\_}{D}}_{1}(t)} = {{\cos \; {\omega\Delta}\; {D_{1}(t)}} + {\sin \; {\omega\Delta}\; {D_{2}(t)}}}} \\{= {{2\; \cos \; {\omega\Delta}\; \cos \; \omega \; t} + {2\; \sin \; {\omega\Delta}\; \sin \; \omega \; t}}}\end{matrix}} & (15)\end{matrix}$

and similarly for D ₂(t). Thus the modified demodulation functions D₁(t) and D ₂(t) can be expressed as a linear combination of basisfunctions. If the time delay Δ can be predicted, then the demodulatorfunctions can be calculated and programmed into the communicationsystem. However, in many cases the time delay Δ is not known or changesover time. As described below, the demodulator functions can bedetermined by system calibration procedures.

When an unknown phase shift (or phase error) is introduced, then theremay be crosstalk in the system. Consider, for example, the situationwhen a phase error φ₁ occurs in the signal S₁(t) and a phase error φ₂occurs in the signal S₂(t). The phase errors can be caused by intrinsicproperties of the components, intrinsic properties of the system,component variations, time delays, etc. In the presence of the phaseerrors:

$\begin{matrix}\begin{matrix}{{{D_{1}(t)}{S(t)}} = {{2\; A_{1}{S_{1}(t)}{\sin \left( {{\omega \; t} + \varphi_{1}} \right)}\sin \; \omega \; t} + {2\; A_{2}{S_{2}(t)}\cos \; \left( {{\omega \; t} + \varphi_{2}} \right)\sin \; \omega \; t}}} \\{= {{2\; A_{1}{S_{1}(t)}\sin \; \omega \; {t\left\lbrack {{\sin \; \omega \; t\; \cos \; \varphi_{1}} + {\cos \; \omega \; t\; \sin \; \varphi_{1}}} \right\rbrack}} +}} \\{{2\; A_{2}{S_{2}(t)}\sin \; \omega \; {t\left\lbrack {{\cos \; \omega \; t\; \cos \; \varphi_{2}} + {\sin \; \omega \; t\; \sin \; \varphi_{2}}} \right\rbrack}}} \\{= {{A_{1}{S_{1}(t)}\cos \; \varphi_{1}} - {A_{1}{S_{1}(t)}\cos \; \varphi_{1}\cos \; 2\; \omega \; t} +}} \\{{{A_{1}{S_{1}(t)}\sin \; \varphi_{1}\sin \; 2\; \omega \; t} + {A_{2}{S_{2}(t)}\cos \; \varphi_{2}\sin \; 2\omega \; t} -}} \\{{{A_{2}{S_{2}(t)}\sin \; \varphi_{2}} + {A_{2}{S_{2}(t)}\sin \; \varphi_{2}\cos \; 2\omega \; t}}\;}\end{matrix} & (16)\end{matrix}$

Thus, after lowpass filtering

Ŝ ₁(t)=A ₁ S ₁(t)cos φ₁ −A ₂ S ₂(t)sin φ₂  (17)

The above equation shows crosstalk because Ŝ₁(t) depends in part oncomponents of S₂(t) when A₂#0 and φ₂≠nπ where n=0, ±1, ±2 . . . .

Similarly,

Ŝ ₂(t)=A ₁ S ₁(t)sin φ₁ +A ₂ S ₂(t)cos φ₂  (18)

The above equations can be expressed in matrix form as:

$\begin{matrix}{\begin{bmatrix}{{\hat{S}}_{1}(t)} \\{{\hat{S}}_{2}(t)}\end{bmatrix} = {\begin{bmatrix}{A_{1}\cos \; \varphi_{1}} & {{- A_{2}}\sin \; \varphi_{2}} \\{A_{1}\sin \; \varphi_{1}} & {A_{2}\cos \; \varphi_{2}}\end{bmatrix}\begin{bmatrix}{S_{1}(t)} \\{S_{2}(t)}\end{bmatrix}}} & (19)\end{matrix}$

After inversion

$\begin{matrix}{\begin{bmatrix}{S_{1}(t)} \\{S_{2}(t)}\end{bmatrix} = {{\frac{1}{\cos \left( {\varphi_{1} - \varphi_{2}} \right)}\begin{bmatrix}\frac{\cos \; \varphi_{2}}{A_{1}} & \frac{\sin \; \varphi_{1}}{A_{1}} \\{- \frac{\sin \; \varphi_{2}}{A_{2}}} & \frac{\cos \; \varphi_{1}}{A_{2}}\end{bmatrix}}\begin{bmatrix}{{\hat{S}}_{1}(t)} \\{{\hat{S}}_{2}(t)}\end{bmatrix}}} & (20) \\{{\begin{bmatrix}{S_{1}(t)} \\{S_{2}(t)}\end{bmatrix} = {{{\frac{1}{\cos \left( {\varphi_{1} - \varphi_{2}} \right)}\begin{bmatrix}\frac{\cos \; \varphi_{2}}{A_{1}} & \frac{\sin \; \varphi_{1}}{A_{1}} \\{- \frac{\sin \; \varphi_{2}}{A_{2}}} & \frac{\cos \; \varphi_{1}}{A_{2}}\end{bmatrix}}\begin{bmatrix}{D_{1}(t)} \\{D_{2}(t)}\end{bmatrix}}{S(t)}}}{Then}} & (21) \\{\begin{bmatrix}{S_{1}(t)} \\{S_{2}(t)}\end{bmatrix} = {\begin{bmatrix}{{\overset{\_}{D}}_{1}(t)} \\{{\overset{\_}{D}}_{2}(t)}\end{bmatrix}{S(t)}}} & (22)\end{matrix}$

Where D _(i)(t) are modified demodulation functions, given by:

$\begin{matrix}{\begin{bmatrix}{{\overset{\_}{D}}_{1}(t)} \\{{\overset{\_}{D}}_{2}(t)}\end{bmatrix} = {\frac{1}{\cos \left( {\varphi_{1} - \varphi_{2}} \right)}\begin{bmatrix}\frac{{\cos \; \varphi_{2}\sin \; \omega \; t} + {\sin \; \varphi_{1}\cos \; \omega \; t}}{A_{1}} \\\frac{{{- \sin}\; \varphi_{2}\sin \; \omega \; t} + {\cos \; \varphi_{1}\cos \; \omega \; t}}{A_{s}}\end{bmatrix}}} & (23)\end{matrix}$

The modified demodulation functions have the form

$\begin{matrix}{{{\overset{\_}{D}}_{i}(t)} = {\sum\limits_{j = 1}^{N}\; {\alpha_{j}{\Phi_{j}(t)}}}} & (24)\end{matrix}$

Choosing the coefficients α_(j) to eliminate crosstalk configures themodulator.

In one embodiment, the coefficients α_(j) can be fixed coefficientscomputed using known properties of the system. However, as systemproperties change over time, such fixed coefficients may lead tounacceptably high levels of crosstalk. Moreover, variations from deviceto device may cause the fixed coefficients to give unacceptably highlevels of crosstalk.

Higher performance (that is, lower crosstalk) can be obtained bycomputing the coefficients as part of a system calibration orinitialization procedure. Such calibration can be performed duringsystem startup (e.g., when the system is turned on, or when the systembegins processing data, etc.) and/or at regular intervals. In oneembodiment, the adjuster algorithm 450 computes the coefficients α_(j)from the calibration data. In one embodiment, the coefficients α_(j) arechosen by making four calibration-type measurements to measure fourparameters ξ₁₁, ξ₁₂, ξ₂₁, and ξ₂₂, where:

ξ₁₁ =Ŝ ₁|_(S) ₁ _(=A,S) ₂ ₌₀ =Aa ₁ cos φ₁  (25)

ξ₁₂ =Ŝ ₁|_(S) ₁ _(=0,S) ₂ _(=B) =−Ba ₂ sin φ₂  (26)

ξ₂₁ =Ŝ ₂|_(S) ₁ _(=A,S) ₂ ₌₀ =Aa ₁ sin φ₁  (27)

ξ₂₂ =Ŝ ₂|_(S) ₁ _(=0,S) ₂ _(=B) =Ba ₂ cos φ₂  (28)

where A and B are amplitudes. Then

$\begin{matrix}{a_{1} = \frac{\sqrt{\xi_{11}^{2} + \xi_{21}^{2}}}{A}} & (29) \\{a_{2} = \frac{\sqrt{\xi_{12}^{2} + \xi_{22}^{2}}}{B}} & (30) \\{{\tan \; \varphi_{1}} = \frac{\xi_{21}}{\xi_{11}}} & (31) \\{{\tan \; \varphi_{2}} = {- \frac{\xi_{12}}{\xi_{22}}}} & (32)\end{matrix}$

From the above equations, it is evident that crosstalk depends only onφ₁ and φ₂. Moreover, φ₁ and φ₂ can be chosen to eliminate crosstalkwithout knowing A or B. This is useful for systems such as pulseoximetry systems where absolute measurements of a channel are difficultor impractical, but where relative measurements (e.g.,channel-to-channel measurements) are practical.

The demodulation signals D_(i)(t) can be generated using the values ofφ₁ and φ₂ from the above equations. Alternatively, the demodulationsignals D_(i)(t) can be generated from quadrature components as:

D ₁(t)=b ₁₁ sin ωt+b ₁₂ cos ωt  (33)

D ₂(t)=b ₂₁ sin ωt+b ₂₂ cos ωt  (34)

where the coefficients b_(ij) are computed from φ₁ and φ₂.

In one embodiment, the demodulation functions are adapted from baselinecoefficients, which are then improved through a calibration orinitialization procedure to produce actual coefficients. The baselinecoefficients are typically obtained from known properties of the system.The actual coefficients are usually relatively close in value to thebaseline coefficients. This provides one way to assess the operationalstatus of the system and to evaluate the calibration procedure. In oneembodiment, if the actual coefficients are too different from thebaseline parameters then it is assumed that the calibration procedurefailed in some manner or that the equipment has failed in some manner,and appropriate measures can be taken (e.g., alert the operator, sound awarning, etc.)

To find the actual coefficients, the demodulation functions areinitially given by:

D ₁(t)=α₁₁ sin ωt+α ₁₂ cos ωt  (35)

D ₂(t)=α₂₁ sin ωt+α ₂₂ cos ωt  (36)

Where the coefficients α_(ij) are the baseline coefficients determinedfrom known or assumed properties of the signal S(t). For example, in oneembodiment α_(ij)=δ_(ij). In one embodiment, where initial estimates areavailable for φ₁ and φ₂, then the values of α_(ij) can be computed asdiscussed above.

The crosstalk reduction obtained using demodulation functions based onthe coefficients α_(ij) can often be improved by computing newcoefficients α _(ij) and corresponding new demodulation functions D_(i)(t) where:

D ₁(t)= α ₁₁ sin ωt+ α ₁₂ cos ωt  (37)

D ₂(t)= α ₂₁ sin ωt+ α ₂₂ cos ωt  (38)

The process of finding the coefficients α _(ij) begins by measuring twodata sets, x₁(t) and x₂(t), as follows:

x ₁(t)=S(t)|_(S) ₁ _(=A,S) ₂ ₌₀  (39)

x ₂(t)=S(t)|_(S) ₁ _(=0,S) ₂ _(=B)  (40)

The data sets x₁(t) and x₂(t) are used to enforce the followingconstraint:

$\begin{matrix}{{\int_{0}^{nT}{{x_{i}(t)}{{\overset{\_}{D}}_{i}(t)}\ }} = 0} & (41)\end{matrix}$

where i=1, 2, n=1, 2, 3 . . . , and T is a time period corresponding toone complete modulation cycle. From the above constraint and thedefinitions of the demodulation functions, it follows that:

$\begin{matrix}{{{{\overset{\_}{\alpha}}_{11}{\int_{0}^{nT}{{x_{1}(t)}\sin \; \omega \; t\ {t}}}} + {{\overset{\_}{\alpha}}_{12}{\int_{0}^{nT}{{x_{1}(t)}\cos \; \omega \; t\ {t}}}}} = 0} & (42) \\{{{{\overset{\_}{\alpha}}_{21}{\int_{0}^{nT}{{x_{2}(t)}\sin \; \omega \; t\ {t}}}} + {{\overset{\_}{\alpha}}_{22}{\int_{0}^{nT}{{x_{2}(t)}\cos \; \omega \; t\ {t}}}}} = 0} & (43)\end{matrix}$

It is convenient to define

$\begin{matrix}{\gamma_{11} = {\int_{0}^{nT}{{x_{1}(t)}\sin \; \omega \; t\ {t}}}} & (44) \\{\gamma_{12} = {\int_{0}^{nT}{{x_{1}(t)}\cos \; \omega \; t\ {t}}}} & (45) \\{\gamma_{21} = {\int_{0}^{nT}{{x_{2}(t)}\sin \; \omega \; t\ {t}}}} & (46) \\{\gamma_{22} = {\int_{0}^{nT}{{x_{2}(t)}\cos \; \omega \; t\ {t}}}} & (47)\end{matrix}$

and to define

$\begin{matrix}{\beta_{ij} = \frac{\gamma_{ij}}{\left\lbrack {\sum\limits_{k}\; \gamma_{ik}^{2}} \right\rbrack^{1/2}}} & (48) \\{{\sum\limits_{k}\; \beta_{ik}^{2}} = 1} & (49)\end{matrix}$

Then

α ₁₁β₁₁+ α ₁₂β₁₂=0  (50)

α ₂₁β₂₁+ α ₂₂β₂₂=0  (51)

In one embodiment, to reduce crosstalk, it is desired to find thecoefficients α _(ij) closest (in the sense of minimizing some specifiederror, such as, for example, a least squared error) to the coefficientsα_(ij) such that the above constraints are satisfied. One solution,obtained by minimizing the least squared error is:

$\begin{matrix}{{\overset{\_}{\alpha}}_{ij} = {\alpha_{ij} - {\left( {\sum\limits_{k}\; {\alpha_{ik}\beta_{ik}}} \right)\beta_{ik}}}} & (52)\end{matrix}$

The term in parentheses can be described as the baseline crosstalk.

One of ordinary skill in the art will recognize that optimizationmethods other than least squares can be used. The solution methods forconfiguration are, for simplicity, described above in terms of atwo-channel system. Using the above teachings, the extension tomulti-channel systems is straightforward.

Although described above in connection with a particular embodiment ofthe present disclosure, it should be understood the description of theembodiment is illustrative of the disclosure and are not intended to belimiting. Although described above in connection with a pulse oximetrysystem wherein a parameter to be measured is the attenuation of red andinfrared light passing through a portion of a subject's body, it shouldbe understood that the method and apparatus described herein can also beused for other measurements where two or more signals are passed througha system to be analyzed. In particular, the present disclosure can beused to demodulate two combined parametric signals responsive to thesystem to be analyzed where the two parametric signals have apredetermined timing relationship between them, as described herein. Thedisclosure can be used in connection with various physiologicalparameter measurement systems, such as, for example, systems thatmeasure blood constituents, blood oxygen carboxyhemoglobin,methemoglobin, glucose, etc. Various modifications and applications mayoccur to those skilled in the art without departing from the true spiritand scope of the invention as defined in the appended claims.

What is claimed is:
 1. A system which measures one or more physiologicalcharacteristics of a living subject non-invasively by measuring pulsinglight of two or more wavelengths after attenuation by body tissue, thesystem comprising: at least one emitter configured to emit light of atleast two or more different wavelengths using a modulation scheme thatcycles the at least two or more different wavelengths, the at least oneemitter positioned to emit light in the direction of a living subject;at least one detector configured to detect the light of the at least twoor more different wavelengths emitted from the at least one detectorafter the light has been attenuated by tissue of the living subject, thedetector further configured to generate a detector signal representativeof the emitted light; a demodulation system configured to demodulate thedetector signal, the demodulation system configured to adjust thedemodulation of the detector signal to reduce crosstalk based, at leastin part, on pre-demodulation signal values.
 2. The system of claim 1,the demodulation system is further configured to adjust the demodulationbased on previously demodulated signal values.
 3. The system of claim 1,wherein the demodulation is adjusted during an initial calibrationperiod.
 4. The system of claim 3, wherein the demodulation is adjustedduring periodic calibration periods during measurement.
 5. The system ofclaim 1, wherein the demodulation system comprises a plurality ofdemodulation coefficients used to demodulate the detector signal.
 6. Thesystem of claim 5, wherein at least one demodulation coefficient is usedfor each of the at least two or more different wavelengths.
 7. Thesystem of claim 2, wherein the previously demodulated signal valuescomprises a plurality of demodulated signal values correspondingrespectively to the at least two or more different wavelengths.
 8. Thesystem of claim 7, wherein each of the plurality of demodulated signalvalues is determined using a demodulation coefficient.
 9. The system ofclaim 1, wherein the demodulation has been diagonalized.
 10. A method ofdemodulating a composite signal generated by applying at least first andsecond periodic pulses of electromagnetic energy to a system having aphysiological parameter to be measured and by non-invasively receivingsignals responsive to said electromagnetic energy after attenuation bybody tissue of a living subject, said signals received as a compositesignal having at least first and second components responsive to said atleast first and second pulses respectively, said method comprising:receiving, at one or more signal processors, a composite signalresponsive to electromagnetic energy of at least first and secondperiodic pulses of electromagnetic energy after attenuation by bodytissue; applying, using the one or more signal processors, a firstdemodulation scheme to said composite signal to generate at least afirst demodulated signal; and automatically adjusting, using the one ormore signal processors, the first demodulation scheme based at least inpart on pre-demodulated signal values.
 11. The method of claim 10,wherein the adjustment reduces crosstalk.
 12. The method of claim 10,further comprising applying a second demodulation scheme to saidcomposite signal to generate a second demodulated signal and adjustingthe second demodulation scheme based at least in part on the seconddemodulated signal.
 13. The apparatus of claim 12, wherein said signalprocessor is further configured to adjust at least one of the first andsecond demodulation schemes in the absence of at least one of the firstand second periodic pulses of electromagnetic energy.
 14. The apparatusof claim 12, wherein said signal processor is further configured toadjust at least one of the first and second demodulation schemes basedon the demodulated signals in order to further reduce crosstalkinterspersed with measurements.
 15. The method of claim 10, whereinadjusting occurs continuously.
 16. The method of claim 10, whereinadjusting occurs interspersed with measurements.
 17. A system fordemodulating a composite signal generated by applying at least first andsecond periodic pulses of electromagnetic energy to a system having aphysiological parameter to be measured and by non-invasively receivingsignals responsive to said electromagnetic energy after attenuation bybody tissue of a living subject, said signals received as a compositesignal having at least first and second components responsive to said atleast first and second pulses respectively, said method comprising:means for receiving a composite signal responsive to electromagneticenergy of at least first and second periodic pulses of electromagneticenergy after attenuation by body tissue; means for applying a firstdemodulation scheme to said composite signal to generate at least afirst demodulated signal; and means for automatically adjusting thefirst demodulation scheme based at least in part on pre-demodulatedsignals.
 18. The method of claim 17, wherein the adjustment reducescrosstalk.
 19. The method of claim 17, further comprising applying asecond demodulation scheme to said composite signal to generate a seconddemodulated signal and adjusting the second demodulation scheme based atleast in part on the second demodulated signal.
 20. The apparatus ofclaim 19, wherein said signal processor is further configured to adjustat least one of the first and second demodulation schemes in the absenceof at least one of the first and second periodic pulses ofelectromagnetic energy.